1. Field of the Invention
The present invention relates to a refraction type semiconductor photo-detector, a semiconductor photo-detection device comprising the semiconductor photo-detector combined with an optical waveguide, and production methods thereof.
2. Description of the Related Art
Conventional refraction type semiconductor photo-detectors have a structure in which, as shown in FIG. 1A, a light incident angled facet 21a is formed across a photo-absorption layer 23a from the top surface, or, as shown in FIG. 1B, an angle begins from a layer right beneath a photo-absorption layer 23b while contacting with the photo-absorption layer 23b (for example, Japanese Patent Application 9-52760 (1997)).
In FIG. 1A, numeral 21a indicates a light incident facet, 22a is a p-InP layer, 23a is an InGaAs photo-absorption layer, 24a is an n-InP layer, 25a is an n-InP layer, 26a is a p electrode, and 27a is an n electrode.
Further, in FIG. 1B, numeral 21b indicates a light incident facet, 22b is a p-InP layer, 23b is an InGaAs photo-absorption layer, 24b is an n-InP layer, 25b is an n-InP substrate, 26b is a p electrode, and 27b is an n-electrode.
Still further, in FIGS. 1A and 1B, numeral 28 indicates an optical fiber as an example of an optical waveguide for conducting the incident light. This optical waveguide 28 is combined with a semiconductor photo-detector shown in FIG. 1A or 1B to construct a semiconductor photo-detection device.
In the production process of above described photo-detector, when forming the reverse-mesa optical incident facet by using wet etching with bromine-methanol or the like, in etching including the photo-absorption layer 23a as a narrow gap or where the photo-absorption layer 23a exists close to the etching, the photo-absorption layer 23a as a narrow-gap is relatively fast in etching speed and, since side etching is liable to occur, an etching irregularity such as uneven side etching tends to generate during deep etching, resulting in a problem of generating fine irregularities or waves on the etching surface.
When the spot size of incident light is large, effect of irregularities or waves is small. However, when an optical beam is focused and applied using a tapered fiber or a lens, this effect becomes conspicuous, the beam is diffused, and focusing of the beam is degraded.
Further, in the prior art structure, in order to obtain a high-speed response, the incident position must be set at the top surface side as possible so that the photo-absorption area is the smallest, when the incident light position is moved down to the substrate side, the photo-absorption part is required to be increased in length to make photo-absorption possible.
As a result, the photo-absorption area is increased resulting in degraded high-speed response characteristics.
Still further, in the above-described prior art semiconductor photo-detector, the chip is formed by using cleavage or the like from the vicinity of the light incident angled facet.
Thus, the chip does not have a guide structure for optical-connection with an optical waveguide such as an optical fiber.
At the time to connect the optical fiber with the photo-detector optically, when the optical beam center of the optical fiber is fitted to the center of the optical-absorption area of the photo-detector, the responsivity becomes maximum, and when the optical beam center of the optical fiber is shifted from the center of the optical-absorption area of the photo-detector, the optical-absorption amount of the photo-detector is decreased, thereby the responsivity is deteriorated.
Although permissible range of the shifting depends on size of the photo-detector and a direction of the shifting, etc., the permissible range is usually several μm in the minimum direction.
As a result, in optical coupling with fibers or the like, fine mechanical adjustment of fiber optical end with an accuracy of several microns is required to a position where the responsivity is the maximum.
Therefore, there is a problem in that when fabricating a module (semiconductor photo-detection device) by combining a photo-detector with a fiber, a very precise positioning technique is necessary, and even a small deviation generates a degraded responsivity or a degraded response speed.
Therefore, in general, one or two lenses are inserted between the photo-detector and the fiber to moderate the positioning accuracy.
However, there is a problem in that the insertion of such a lens system leads to increases in the number of parts or fabrication steps resulting in an increase in module cost.
Further, there is a report of a structure in which to perform good optical coupling with the fiber without using the above lens system, the photo-detector is mounted on an optical fiber holding substrate having a V-shaped groove comprising silicon or the like. However, in this construction, it is required that the optical fiber holding substrate and the photo-detector be connected in high mechanical precision, which requires a high-precision positioning technique, and a small deviation generates reduction of responsivity or response speed.
Still further, even when the lens system is inserted as described above, there is some distance deviation between the device and the lens system during positioning, which is a cause of deviation in responsivity in a device with a small misalignment tolerance.
Yet further, in the above-described prior art semiconductor photo-detector, the electrodes 26a and 26b of the upper layer are, in general, alloyed with the semiconductor layer by heat treating metals such as AuZnNi for the case of p-type or AuGeNi for the case of n-type to form ohmic electrodes.
By virtue of such alloying, fine irregularities are generated between the electrodes 26a and 26b and the p-InP layers 22a and 22b as semiconductor, even if refracted light reaches here, it is diffuse reflected or absorbed by the electrode metal itself, and the electrode part is small in light reflectivity.
Therefore, although the thicknesses of the photo-absorption layers 23a and 23b can be reduced by increasing the effective absorption length through which the refracted light transits diagonally with respect to the layer thickness direction which is a characteristic of the refraction type semiconductor photo-detector, to obtain a sufficiently large responsivity, refracted light to the photo-absorption layers 23a and 23b is required to be sufficiently absorbed by one transit, there has been a limitation in reducing the thickness of the photo-absorption layers 23a and 23b. 
As a result, a transit time of carriers transitting the photo-absorption layers 23a and 23b is a limitation factor of response speed of the semiconductor photo-detector, and there is a problem in that an ultra-high speed and high responsivity device cannot be fabricated.
Yet further, in the prior art refraction type semiconductor photo-detection device, for example, as shown in FIG. 1A, the light incident facet 21a of the refraction type semiconductor photo-detector and an optical waveguide such as a single mode optical fiber are disposed in opposition, and a gas such as air or an inert gas is filled in between.
Here, since the gas has a refractive index of nearly 1 and the refractive index of the photo-detector material is constant, the refraction angle at the light incident facet 21a is determined only by the reverse-mesa angle.
In general, in the production process of refraction type semiconductor photo-detector, when fabrication is made by determining the reverse-mesa angle, mesa angles of devices in the same wafer become all in line with each other.
Since the refraction type semiconductor photo-detector utilizes an increase in effective absorption length by transiting light the optical absorption layer by refraction, the refraction angle is determined equally in the prior art, and the effective absorption length is also constant.
Therefore, there is a problem in that to change the effective absorption length according to various applications, wafers of different mesa angles or different absorption layer thicknesses are required to be prepared to change the refraction angle.